This discussion paper will assess the intellectual trajectory of Kenneth Waltz in the period 1959–79. The paper will argue that Waltz's oeuvre is best understood via connection to how the human mind was being conceptualised and explored in the mid-twentieth century. Over the course of these 20 years Waltz published three of his most significant single-author books. The third of these was Theory of International Politics (1979), a text still definitive for the field of academic International Relations in the present day. However, Waltz's IR magnum opus was built up over the course of twenty years via a gradual fine-tuning of his scholarly approach. Where Waltz began in 1959 with wide-ranging criticism of how international politics had been assessed by his peers and intellectual forebears, by 1979 Waltz had formulated a strident and detailed proposal for how the field should direct its investigations into the dynamics of the international world. The paper will unpack three of Waltz's key texts – Man, the State, and War (1959), Foreign Policy and Democratic Politics (1967), and Theory of International Politics (1979) – in this regard. It will conclude with two main concluding ideas, one specific to Waltz and one more general: 1) Waltz's intellectual trajectory was contingent upon core psychological ideas and trends that shaped the social sciences in the post-WWII world, and 2) unpicking the psychological underpinnings of different International Relations theories helps to better understand them, both historically and for the purposes of comparative debate.